Cushion load control device and press machine having cushion load control device

ABSTRACT

A cushion load control device for a press machine includes a hydraulic cylinder  2  forming a die cushion, a hydraulic control valve  4  connected to the hydraulic cylinder  2 , a hydraulic source  6  connected to the hydraulic cylinder  2  via the hydraulic control valve  4 , an opening degree controlling part  11  for outputting a control signal for controlling an opening degree of the hydraulic control valve  4  on the basis of the set pressure value of the hydraulic cylinder, a speed detection unit  15  for detecting the lowering speed of the die cushion, and a signal correction part  18  for correcting the value of the control signal in accordance with the set pressure value and the lowering speed value detected by the speed detection unit  15.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention relates to a cushion load control device forcontrolling a load of a die cushion and a press machine having thecushion load control device.

2. Description of the Related Art

In a press machine, for instance, when a crank shaft is driven by amotor to rotate, the crank shaft operates so as to convert rotatingmotion into up-and-down motion, thereby moving up and down a slideconnected to the crank shaft.

When lowering the slide, press operation of the press machine is carriedout by inserting a workpiece between an upper die fixed to a lowersurface of the slide and a lower die disposed below the slide.

A cushion mechanism is provided below the slide to receive a load of thelowered slide (i.e., the upper die) during the press operation.

FIG. 1 shows the cushion mechanism. The cushion mechanism is fixed to abed 28 as a lower structural body of the press machine, and includes abolster plate 45 having an upper surface to which the lower die isfixed, a blank holder 46 which is provided in the lower die 43, aplurality of cushion pins 47 which support the blank holder 46 from thedownside, a pin plate 49 which supports the cushion pins 47 from thedownside, and a cushion pad 14 which supports the pin plate 49 from thedownside.

The cushion mechanism further includes a hydraulic cylinder 2 and an aircylinder 53 for receiving the press load acting on the cushion pad 14.

The die cushion is constructed by the blank holder 46, the cushion pins47, the pin plate 49, the cushion pad 14, and the hydraulic cylinder 2.

When the slide 54 is lowered so that an upper die 56 comes into contactwith the blank holder 46 during the press operation, the pin plate 49 islowered and the cushion pad 14 being in contact with the pin plate 49 isaccordingly lowered.

At this time, the press load applied from the upper die 56 to thecushion pad 14 is supported by the hydraulic cylinder 2 and the aircylinder 53 disposed in the lower portion of the cushion pad 14.

A set pressure value of the hydraulic cylinder 2 is set in advance withrespect to a rotation angle of the crank shaft so as to obtain anappropriate cushion performance. For instance, the set pressure value isset to a constant value.

The press machine further includes a cushion load control device. Asshown in FIG. 1, the cushion load control device includes the hydrauliccylinder 2, a servo valve 4 which is connected to the hydraulic cylinder2, a hydraulic tank 6 which is connected to the hydraulic cylinder 2through the servo valve 4, a pressure transducer 12 which detects ahydraulic pressure of the hydraulic cylinder 2, and an opening degreecontrolling part 11 which compares the set pressure value with apressure detection value obtained by the pressure transducer 12 andcontrols an opening degree of the servo valve 4 so that a hydraulicpressure of the hydraulic cylinder 2 becomes the set pressure value.

However, when a feedback control is carried out by the opening degreecontrolling part 11, a lowering speed of the slide 54 varies inaccordance with the rotation angle of the crank shaft, therebydeteriorating a control performance. Particularly, since the loweringspeed of the slide 54 decreases at a position around BDC (bottom deadcenter), the pressure of the hydraulic cylinder 2 becomes largelysmaller than the set pressure value.

For this reason, in Patent Document 1, the following control is carriedout.

The cushion load control device disclosed in Patent Document 1 detectsthe lowering speed of the slide and carries out a control correction sothat the servo valve is operated toward its closing position when thelowering speed of the slide becomes smaller as the slide comes closer toBDC. Hence, the hydraulic pressure of the hydraulic cylinder iscontrolled to be closer to the set pressure value.

[Patent Document 1] Japanese Laid-open Patent Publication No. H04(1992)-172199 ‘Pressure controlling Apparatus for NC die cushion’

However, although the cushion load control device disclosed in PatentDocument 1 corrects the opening degree of the servo valve in accordancewith the lowering speed of the slide, if the control disclosed in PatentDocument 1 is carried out when a set load of the die cushion variesdepending upon difference in the type of a workpiece as a press object,the hydraulic pressure of the hydraulic cylinder deviates from the setpressure value. As a result, a problem arises in that an actual load,which acts on the die cushion deviates from the set load value.

SUMMARY OF THE INVENTION

Therefore, a first object of the invention is to provide a cushion loadcontrol device capable of automatically preventing an actual load actingon a die cushion from deviating from a set load value even when the setload value of the die cushion varies depending on a difference in thetype of a workpiece as a press object.

In addition, a second object of the invention is to provide a pressmachine provided with a cushion load control device capable ofautomatically preventing an actual load acting on the die cushion fromdeviating from the set value even when the set load value of the diecushion varies.

According to the invention, in order to attain the first object, thereis provided a cushion load control device for a press machine forcontrolling a press load acting on a die cushion disposed below a slideoperable to be moved up and down, the cushion load control deviceincluding: a hydraulic cylinder which constitutes a die cushion; ahydraulic control valve configured to be connected to the hydrauliccylinder; a hydraulic source configured to be connected to the hydrauliccylinder via the hydraulic control valve; an opening degree controllingpart configured to output a control signal for controlling an openingdegree of the hydraulic control valve on the basis of a set pressurevalue of the hydraulic cylinder; a speed detection unit configured todetect a lowering speed of the die cushion; and a signal correction partconfigured to correct a value of the control signal in accordance withthe set pressure value and a lowering speed value detected by the speeddetection unit, wherein the opening degree of the hydraulic controlvalve is adjustably controlled on the basis of the control signalcorrected by the signal correction part.

With the described configuration, since the signal control part correctsthe control signal by taking into consideration not only the loweringspeed of the die cushion but also the set pressure value of thehydraulic cylinder, it is possible to automatically prevent thehydraulic pressure of the hydraulic cylinder from deviating from the setpressure value even when the set load of the die cushion (i.e., the setpressure value of the hydraulic cylinder) largely varies.

According to a preferred embodiment of the invention, the signalcorrection part corrects the value of the control signal so that theopening degree of the hydraulic control valve is more reduced as the setpressure value becomes larger.

The hydraulic pressure of the hydraulic cylinder generally has atendency of becoming smaller than the set pressure value at a positionaround BDC as the set pressure value becomes larger and becoming largerthan the set pressure value at a position around BDC as the set pressurevalue becomes smaller.

Nevertheless, in this embodiment, since the value of the control signalis corrected so that the opening degree of the hydraulic control valveis more reduced as the set pressure value becomes larger, it is possibleto suppress the tendency.

According to a further preferred embodiment of the invention, the signalcorrection part corrects the value of the control signal so that theopening degree of the hydraulic control valve is more reduced as thelowering speed of the die cushion is slower.

The hydraulic pressure of the hydraulic cylinder has a general tendencysuch that the hydraulic pressure is reduced more than the set pressurevalue as the lowering speed of the die cushion is slower.

Nevertheless, in this embodiment, since the value of the control signalis corrected so that the opening degree of the hydraulic control valveis reduced as the lowering speed of the die cushion becomes slower, itis possible to correct a variation in hydraulic pressure due to avariation in the set pressure value and a variation in the loweringspeed of the die cushion.

According to the invention, in order to attain the second object, thereis provided a press machine including: a motor; a rotation element whichis rotationally driven by the motor; a converting mechanism configuredto covert rotating motion of the rotation element into reciprocatingmotion; a slide configured to be connected to the converting mechanismto thereby perform a reciprocating motion; and a cushion load controldevice configured to control a press load acting on a die cushiondisposed below the slide, wherein the cushion load control deviceincludes a hydraulic cylinder which constitutes a die cushion; ahydraulic control valve configured to be connected to the hydrauliccylinder; a hydraulic source configured to be connected to the hydrauliccylinder via the hydraulic control valve; an opening degree controllingpart configured to output a control signal for controlling an openingdegree of the hydraulic control valve on the basis of a set pressurevalue of the hydraulic cylinder; a speed detection unit configured todetect a lowering speed of the die cushion; and a signal correction partconfigured to correct a value of the control signal in accordance withthe set pressure value and a lowering speed value detected by the speeddetection unit, and wherein the opening degree of the hydraulic controlvalve is controlled on the basis of the control signal corrected by thesignal correction part.

In the press machine with the described configuration, since the signalcontrol part corrects the control signal by taking into considerationnot only the lowering speed of the die cushion but also the set pressurevalue of the hydraulic cylinder, it is possible to automatically preventthe hydraulic pressure of the hydraulic cylinder from deviating from theset pressure value even when the set load of the die cushion (i.e., theset pressure value of the hydraulic cylinder) largely varies.

According to the invention, in order to attain the first object, thereis provided a cushion load control device for a press machine, whichcontrols a press load acting on a die cushion disposed below a slideoperable to be moved up and down, the cushion load control deviceincluding: a servo motor; a power transmission unit configured toconvert rotating motion of the servo motor into up-and-down motion so asto move up and down the die cushion; a position control part configuredto output a control signal for controlling a height of the die cushionon the basis of a set load value of the die cushion; a speed detectionunit which detects a lowering speed of the die cushion; and a signalcorrection part configured to correct a value of the control signal inresponse to the set load value and a lowering speed value detected bythe speed detection unit, wherein the height of the die cushion isadjustably controlled on the basis of the control signal corrected bythe signal correction part.

With the foregoing configuration, since the control signal is correctedon the basis of the set load value and the lowering speed of the diecushion, it is possible to automatically prevent the actual load actingon the die cushion from deviating from the set load value in accordancewith a variation in the set load value and the lowering speed of the diecushion.

As described above, according to the invention, it is possible toautomatically prevent the hydraulic pressure from deviating from the setpressure value even when the set load of the die cushion varies.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view illustrating a general configuration of aload control device and a cushion mechanism of a press machine accordingto a conventional art.

FIG. 2 is a diagrammatic view illustrating a configuration of a cushionload control device according to a first embodiment of the invention.

FIG. 3A is a diagram illustrating an actual hydraulic pressure value anda set pressure value when a hydraulic control is carried out on thebasis of a variation in lowering speed of a cushion pad.

FIG. 3B is another diagram illustrating an actual hydraulic pressurevalue and a set pressure value when a hydraulic control is carried outon the basis of a variation in lowering speed of a cushion pad.

FIG. 3C is another diagram illustrating an actual hydraulic pressurevalue and a set pressure value when a hydraulic control is carried outon the basis of a variation in lowering speed of a cushion pad.

FIG. 4A is a diagram illustrating the actual hydraulic pressure valueand the set pressure value when the hydraulic control is carried out onthe basis of a variation in set pressure value and lowering speed of thecushion pad.

FIG. 4B is another diagram illustrating the actual hydraulic pressurevalue and the set pressure value when the hydraulic control is carriedout on the basis of a variation in set pressure value and lowering speedof the cushion pad.

FIG. 4C is another diagram illustrating the actual hydraulic pressurevalue and the set pressure value when the hydraulic control is carriedout on the basis of a variation in set pressure value and lowering speedof the cushion pad.

FIG. 5 is a diagrammatic view illustrating a configuration of a cushionload control device according to a second embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Hereinafter, preferred embodiments of the invention will be described indetail with reference to the accompanying drawings. In addition, in therespective drawings, the same reference numerals are given to the samecomponents and the repetitive description thereof will be omitted.

First Embodiment

A press machine includes a motor, a crank shaft as a rotation member(element) which is rotationally driven by the motor so as to convertrotating motion into reciprocating motion, a slide which is connected tothe crank shaft so as to move up and down, an upper die which is fixedto a lower surface of the slide, and a cushion mechanism which isdisposed below the slide and to which a load is applied from the upperdie when the slide descends. In addition, the crank shaft as therotation member and mechanical components connected to the crank shaftconfigure a converting mechanism which converts the rotating motion ofthe rotation member into the reciprocating motion of the slide, but theconverting mechanism may be appropriately configured as, for instance, acam or the like.

Since the cushion mechanism has the same configuration as that shown inFIG. 1, the description will be omitted. In such a cushion mechanism, apress load which is applied from the slide (i.e., upper die) to acushion pad 14 during a press operation is set by a hydraulic pressurevalue of a hydraulic cylinder 2.

The press machine is provided with a cushion load control device whichperforms a control for maintaining the hydraulic pressure value of thehydraulic cylinder 2 at a set pressure value during the press operation.

FIG. 2 shows the configuration of a cushion load control device 10according to the first embodiment. As shown in FIG. 2, the cushion loadcontrol device 10 includes the hydraulic cylinder 2, a servo valve 4which is connected to the hydraulic cylinder 2, a hydraulic tank 6 whichis connected to the hydraulic cylinder 2 via the servo valve 4, a loadcommand part 8 which is capable of outputting a set pressure value ofthe hydraulic cylinder 2 in accordance with press conditions input, forexample, by an operator, and an opening degree controlling part 11 whichis capable of outputting a control signal for controlling an openingdegree of the servo valve 4 on the basis of the set pressure value thatis input by the load command part 8.

In addition, the hydraulic control valve is configured as the servovalve 4, but may be configured as other appropriate elements. Thehydraulic source is configured as the hydraulic tank 6, but may beconfigured as other appropriate elements.

The opening degree controlling part 11 outputs control a signal formaintaining the hydraulic pressure value of the hydraulic cylinder 2 atthe set pressure value on the basis of a difference obtained bycomparing the set pressure value that is output by the load command part8 and a hydraulic pressure detection value that is output by a pressuretransducer 12 (PT) as a hydraulic pressure sensor for measuring ahydraulic pressure value of the hydraulic cylinder 2.

The control signal is input to the servo valve 4 via a later-describedsignal correction part 18 and a limiter 9. The servo valve 4 adjustablycontrols the opening degree on the basis of the input control signal.For instance, the control signal may be a voltage signal, and theopening degree of the servo valve 4 is proportional to the voltagevalue.

In addition, since the press load is applied from the slide to thehydraulic cylinder 2 during the press operation, the hydraulic pressurevalue of the hydraulic cylinder 2 is larger than that of the hydraulictank 6. At the press operation in which a piston of the hydrauliccylinder 2 descends, the opening degree of the servo valve 4 iscontrolled so as to control a flow amount from the hydraulic cylinder 2to the hydraulic tank 6, thereby maintaining the hydraulic pressurevalue of the hydraulic cylinder 2 at the set pressure value.

According to this embodiment, the cushion load control device 10includes a speed sensor 15 which detects a lowering speed of the diecushion.

The speed sensor 15 may be configured as, for instance, an element orunit for measuring a lowering speed of the die cushion on the basis of adetection value obtained by a speed sensor for measuring a speed or astroke sensor for measuring a stroke of the cushion pad 14 or the slide.Alternatively, the speed sensor 15 may be configured as a member forcalculating the lowering speed of the die cushion on the basis of acrank shaft rotary angle detection value obtained by a rotary encoderand a predetermined relationship between crank shaft rotary angles andthe lowering speeds of the cushion pad 14.

In addition, a speed detection unit for measuring the lowering speed ofthe die cushion is configured as the speed sensor 15, but may beconfigured as other appropriate members.

Hereinafter, a case will be described in which the lowering speed of thedie cushion is detected on the basis of a speed of the cushion pad 14.

The cushion load control device 10 further includes the signalcorrection part 18 which corrects a value of the control signal outputby the opening degree controlling part 11 in accordance with the setpressure value output by the load command part 8 and the lowering speedof the cushion pad 14 detected by the speed sensor 15.

As shown in FIG. 2, the signal correction part 18 includes a calculationpart 18 a and an addition part 18 b. The calculation part 18 acalculates a correction value on the basis of the set pressure valueoutput by the load command part 8 and the speed detection value of thecushion pad 14 detected by the speed sensor 15.

The addition part 18 b outputs the corrected control signal by addingthe correction value output by the calculation part 18 a to the value ofthe control signal output by the opening degree controlling part 11.

First, for the purpose of comparison, a case will be described in whichthe signal correction part 18 corrects the control signal on the basisof the speed detection value among the set pressure value and the speeddetection value of the cushion pad 14 detected by the speed sensor 15.Then, another case will be described in which the control signal iscorrected on the basis of both the speed detection value and the setpressure value.

The signal correction part 18 corrects the value of the control signalso that the opening degree of the servo valve 4 is reduced as thelowering speed of the cushion pad 14 becomes slower. In this case, thecorrection value may be set so that the correction value of the controlsignal are proportional to the lowering speed of the cushion pad 14.

In the graphs of FIGS. 3A through 3C, the ordinate indicates thehydraulic pressure value of the hydraulic cylinder 2, and the abscissaindicates the crank shaft rotation angle. The crank shaft rotation angleindicates the actual rotation angle of the crank shaft. The slidereciprocates once between TDC (top dead center) and BDC (bottom deadcenter) during one complete rotation of the crank shaft. In FIGS. 3Athrough 3C, the crank shaft rotation angle when the slide is located atTDC is set to be 0 degree, the crank shaft rotation angle increases asthe slide approaches BDC, and when the slide arrives at BDC, the crankshaft rotation angle is set to be 180 degree. The crank shaft rotationangle further increases as the slide comes close to TDC after passingthrough BDC, and when the slide arrives at TDC, the crank shaft rotationangle is set to be 360 degree, which is the same rotational position asthat at 0 degree. In addition, in the drawings of FIGS. 3A through 3C,the dashed line indicates the set pressure value that is output by theload command part 8, and the solid line indicates an actual hydraulicpressure value of the hydraulic cylinder 2 when the control signal iscorrected on the basis of the lowering speed of the cushion pad 14. Inthe example shown in FIGS. 3A through 3C, when the crank shaft rotationangle is at 120 degree, the cushion pad 14 starts to descend, and whenthe crank shaft rotation angle is at 180 degree, the slide is located atBDC, thereby allowing the piston of the hydraulic cylinder 2 to be inits most pressed state.

When P_(R) denotes a rated pressure drop of the servo valve 4, P_(T)denotes a pressure value of the hydraulic tank 6, and P_(C) denotes aset pressure value of the hydraulic cylinder 2, FIG. 3A indicates a casewhere P_(R)≈P_(C)−P_(T), FIG. 3B indicates a case whereP_(R)<<P_(C)−P_(T), and FIG. 3C indicates a case whereP_(R)>>P_(C)−P_(T).

As will be understood from the indication of FIGS. 3A through 3C, in thecase where P_(R)≈P_(C)−P_(T), the actual hydraulic pressure value isidentical with the set pressure value except for a press start time.However, in the case where P_(R)<<P_(C)−P_(T) due to an increase in setpressure value, an overshoot amount that the actual hydraulic pressurevalue goes past the set pressure value becomes larger at the press starttime, and the actual pressure value becomes smaller than the setpressure value at a position around BDC.

In addition, in the case where P_(R)>>P_(C)−P_(T), the overshoot amountthat the actual hydraulic pressure value goes past the set pressurevalue becomes smaller at the press start time, but the actual hydraulicpressure value becomes larger than the set pressure value at a positionaround BDC.

In this way, when the control signal correction is carried out on thebasis of the lowering speed of the cushion pad 14, in the case whereP_(R)≈P_(C)−P_(T), the actual hydraulic pressure value is identical withthe set pressure value except for a start time of the press operation.However, when the set pressure value is largely deviated fromP_(C)=P_(T)+P_(R), the actual pressure value goes far from the setpressure value.

For this reason, according to this embodiment, the signal correctionpart 18 corrects the control signal on the basis of the set pressurevalue of the hydraulic cylinder 2 as well as the lowering speed of thecushion pad 14.

A case will now be described hereinbelow in which the control signal iscorrected on the basis of the set pressure value and the lowering speedof the cushion pad 14.

As shown in FIG. 2, the calculation part 18 a calculates the correctionvalue of the control signal on the basis of the set pressure value thatis output by the load command part 8 as well as the lowering speed ofthe cushion pad 14 that is output by the speed sensor 15.

The signal correction part 18 corrects the control signal so that theopening degree of the servo valve 4 is more reduced as the set pressurevalue increases.

When the value of the control signal is corrected in this way, theresult shown in FIGS. 4A through 4C is obtained. FIG. 4A indicates acase where P_(R)≈P_(C)−P_(T), FIG. 4B indicates a case whereP_(R)<<P_(C)−P_(T), and FIG. 4C shows a case where P_(R)>>P_(C)−P_(T).

As will be clearly understood from FIGS. 4A through 4C, the actualhydraulic pressure value is identical with the set pressure value exceptfor the start time of the press operation.

Such a correction value may be proportional to, for instance, theFormula 1, below.

{P_(R)/(P_(C)−P_(T))}^(1/2)×v  [Formula 1]

In the Formula 1, P_(R), P_(C), and P_(T) are the same as describedabove, and V denotes the lowering speed of the cushion pad 14.

In addition, in this example, the speed V corresponds to the loweringspeed of the cushion pad 14 from the time when the cushion pad 14 startsto descend to the time when the cushion pad 14 arrives at BDC, andbecomes 0 when the cushion pad 14 arrives at BDC. After BDC, the speed Vis maintained at 0.

Accordingly, the correction value increases, as shown in the Formula 1,as the lowering speed V becomes faster.

As understood from the Formula 1, the correction value is proportionalalso to the Formula 2.

{P_(R)/(P_(C)−P_(T))}^(1/2)  [Formula 2]

In this way, as shown in the Formula 2, since the correction value isset so as to decreases as the set pressure value increases inconsideration of the rated pressure drop P_(R) of the servo valve 4 andthe pressure P_(T) of the hydraulic tank 6, it is possible to carry outa control in accordance with a designed reference value of the servovalve 4.

In addition, in a case where the hydraulic cylinder 2 is connected tothe hydraulic tank 6 through a plurality of servo valves 4 arranged inparallel, and opening degrees of these servo valves 4 are controlled ina synchronized manner, it is desirable to consider the number of servovalves 4.

It is desirable to consider effective area of the hydraulic cylinder 2or rated flow amount of the servo valve 4.

Formula 3 indicates a correction value in consideration of such factors.

(1/n)×(A/Q_(R))×{P_(R)/(P_(C)−P_(T))}×V  [Formula 3]

In the Formula 3, n denotes the number of servo valves 4, A denoteseffective area of a lower chamber of the hydraulic cylinder, and Q_(R)denotes rated flow amount of the servo valve 4.

In terms of the correction value indicated in the Formula 3, it ispossible to control the opening degree in consideration of the number nof the servo valves 4, the effective area A of the hydraulic cylinder 2,and the rated flow amount Q_(R) of the servo valve 4.

Second Embodiment

FIG. 5 shows a configuration of the cushion load control device 20according to the second embodiment of the invention. The cushionmechanism of the second embodiment is provided with a servo motor 26instead of the hydraulic cylinder 2 shown in FIG. 2. The servo motor 26moves up and down the cushion pad 14 so as to adjust a load that acts onthe cushion pad 14. Hereinafter, different parts from the firstembodiment will be described. The other parts may be same as or similarto those of the first embodiment.

The cushion load control device 20 according to the second embodimentincludes a load cell 24 as a load measuring part which is fixed to thelower surface of the cushion pad 14 so as to detect a load acting on thecushion pad 14 during the press operation, a rod 25 which is coupled tothe lower portion of the load cell 24 and is driven to move up and downto move up and down the cushion pad 14, and the servo motor 26 whichdrives a ball nut 29 to be rotated via a transmission belt 27.

The ball nut 29 is mounted, for instance, at a press machine bed 28 soas to be rotatable and to be immovable in a vertical direction. A ballscrew 32 is formed in the rod 25 so as to be in screw-engagement with afemale screw of the ball nut 29. Accordingly, when the ball nut 29 isdriven to rotate, the rod 25 is moved up and down. In addition, a powertransmission means (unit) is configured by the transmission belt 27 andthe ball nut 29, but may be configured as other appropriate members.

The cushion load control device 20 includes the load command part 8which outputs the set load value and a position control part 34 whichoutputs control signal for rotating the servo motor 26 on the basis ofthe set load value that is input by the load command part 8.

The position control part 34 compares the set load value with a loaddetection value detected by the load cell 24, and performs a feedbackcontrol so that the load acting on the cushion pad 14 becomes the loadinstruction value (the set load value) on the basis of the comparisonresult.

In the same way as the first embodiment, the cushion load control device20 further includes the speed sensor 15 which detects the lowering speedof the cushion pad 14.

In addition, the cushion load control device 20 further includes thecalculation part 18 a which calculates the correction value of thecontrol signal on the basis of the set load value that is output by theload command part 8 and the lowering speed value detected by the speedsensor 15.

Then, the addition part 18 b of the cushion load control device 20corrects the control signal by adding the correction value that is inputby the calculation part 18 a to the value of the control signal that isinput by the position control part 34.

A motor drive part 36 receives the corrected control signal and allowscurrent to flow to the servo motor 26 via the limiter 9 for apredetermined time so as to drive the servo motor 26 to rotate by therotation angle in accordance with the corrected control signal.

In this way, since the control signal is corrected on the basis of theset load value and the lowering speed of the cushion pad 14, it ispossible to restrict the actual load acting on the cushion pad 14 fromdeviating from the set load value in accordance with a variation in theset load value and the lowering speed of the cushion pad 14.

Other Embodiments

The invention is not limited to the preferred embodiments, but may be,of course, modified variously in the scope without departing from thespirit of the invention.

1. A cushion load control device for a press machine for controlling apress load acting on a die cushion disposed below a slide capable ofperforming an up-and-down motion, the cushion load control devicecomprising: a hydraulic cylinder which constitutes a die cushion; ahydraulic control valve configured to be connected to the hydrauliccylinder; a hydraulic source configured to be connected to the hydrauliccylinder via the hydraulic control valve; an opening degree controllingpart configured to output a control signal for controlling an openingdegree of the hydraulic control valve on the basis of a set pressurevalue of the hydraulic cylinder; a speed detection unit configured todetect a lowering speed of the die cushion; and a signal correction partconfigured to correct a value of the control signal in accordance withthe set pressure value and a lowering speed value detected by the speeddetection unit, wherein the opening degree of the hydraulic controlvalve is controlled on the basis of the control signal corrected by thesignal correction part.
 2. The cushion load control device according toclaim 1, wherein the signal correction part corrects the value of thecontrol signal in such a manner that the opening degree of the hydrauliccontrol valve is reduced as the set pressure value is larger.
 3. Thecushion load control device according to claim 1, wherein the signalcorrection part corrects the value of the control signal in such amanner that the opening degree of the hydraulic control valve is reducedas the lowering speed of the die cushion becomes slower.
 4. A pressmachine comprising: a motor; a converting mechanism including a rotationelement driven by the motor to perform rotating motion and configured tocovert the rotating motion into reciprocating motion; a slide configuredto be connected to the converting mechanism to perform reciprocatingmotion; and a cushion load control device which controls a press loadacting on a die cushion disposed below the slide, wherein the cushionload control device comprises a hydraulic cylinder which constitutes adie cushion; a hydraulic control valve configured to be connected to thehydraulic cylinder; a hydraulic source which is connected to thehydraulic cylinder via the hydraulic control valve; an opening degreecontrolling part configured to output a control signal for controllingan opening degree of the hydraulic control valve on the basis of a setpressure value of the hydraulic cylinder; a speed detection unitconfigured to detect a lowering speed of the die cushion; and a signalcorrection part configured to correct a value of the control signal inaccordance with the set pressure value and a lowering speed valuedetected by the speed detection unit, and wherein the opening degree ofthe hydraulic control valve is controlled on the basis of the controlsignal corrected by the signal correction part.
 5. A cushion loadcontrol device for a press machine for controlling a press load actingon a die cushion disposed below a slide capable of performing anup-and-down motion, the cushion load control device comprising: a servomotor; a power transmission unit configured to convert rotating motionof the servo motor into an up-and-down motion to thereby move up anddown the die cushion; a position control part configured to output acontrol signal for controlling height of the die cushion on the basis ofa set load value of the die cushion; a speed detection unit configuredto detect a lowering speed of the die cushion; and a signal correctionpart configured to correct a value of the control signal in accordancewith the set load value and a lowering speed value detected by the speeddetection unit, wherein the height of the die cushion is controlled onthe basis of the control signal corrected by the signal correction part.6. The cushion load control device according to claim 2, wherein thesignal correction part corrects the value of the control signal in sucha manner that the opening degree of the hydraulic control valve isreduced as the lowering speed of the die cushion becomes slower.